The invention relates generally to electronic fuel control systems for compression ignition engines, and more particularly to fuel injection control systems for compression ignition engines having cylinders with large displacement volumes such as locomotive or marine type engines.
Currently, no variable injection timing systems for diesel engines are known to be commercially available for locomotive applications. Known locomotive engines typically have fixed injection timing through a governor and mechanical linkages which actuate a series of fuel delivery devices simultaneously. Fuel amounts and/or timing of fuel injection are generally predetermined for any given engine operating point and typically cannot be modified for varying conditions. Fuel delivery systems may include pump-line-nozzle configurations or unit injection configurations.
Strict emission requirements will soon be imposed on the locomotive engine industry to reduce exhaust fuel emissions to suitable levels. Although electronic fuel control systems for reducing fuel emissions have been developed for diesel truck engines, these types of systems are generally not suited to the unique designs of much larger diesel engines.
For example, the single cylinder displacement for a large sixteen cylinder locomotive diesel engine may be on the order of 11 liters whereas the single cylinder displacement for a typical diesel truck may be on the order of only 2 liters per cylinder. Therefore a single cylinder for a large locomotive engine may easily be more than five times larger than that of a large diesel truck. This generally translates into very different design constraints since high injection pressure levels (on the order of 10-20 k.p.s.i.) are required in conjunction with much higher volume fuel flow rate ranges (100-1600 mm.sup.3 /stroke) to effectuate proper combustion in the larger locomotive engine.
A problem arises with such large engines since both low fuel volume demand conditions, such as during idle (on the order of 100 mm.sup.3 /stroke), and high volume demand conditions (on the order of 1600 mm.sup.3 /stroke) must be accommodated during normal operating conditions. Therefore the fuel delivery system must be capable of providing a wide range of fuel flow volumes at high injection pressures. Accommodating such requirements becomes increasingly difficult where the fuel delivery mechanisms use a constant displacement type system as known in the art. These systems typically fill a pumping chamber with the same amount of fuel irrespective of the actual fuel necessary for the injector. During low fuel volume demand conditions, excess fuel from the pumping chamber must be spilled or recirculated to a supply reservoir. Consequently both high spill volumes (e.g., 1500 mm.sup.3 /stroke when the fuel volume demand is only 100 mm.sup.3 /stroke) and high pressures must be accommodated by the fuel delivery mechanism. Currently no electronically controllable fuel delivery mechanisms are known to be commercially available which may accommodate such high spill volumes and provide such high injection pressures.
Other differences also impact the type of fuel injection system which may be employed on larger compression ignition engines. For example, locomotive engines are typically designed to maintain governor stability e.g., provide a relatively constant speed output to provide a steady power generating source for large DC motors used to propel the wheels. Powering large DC motors also causes additional electromagnetic interference not typically found in truck applications.
Also, large locomotive engines encounter radical load changes due to switching of large auxiliary loads such as compressor loads, fan loads, and "hotel" power loads (an alternator for generating 110 V at 60 hz) for passenger train applications. Driving such loads or turning off such loads can result in load changes on the order of 500 horsepower at any instant.
Larger engines also typically generate much larger vibrations. It has been a problem to provide accurate engine speed information using such devices as magnetic pick-up sensors since vibrations cause false triggering and typical notches in flywheels only provide coarse and noisy signals typically insufficient for accurate electronically controlled injection timing systems. Hence regular timing components and other electrical devices cannot typically withstand such severe operating conditions.
Another design consideration generally unique to such larger engines, is lower engine speeds (RPM) and reduced chamber air movement. Smaller engines typically operate at engine speeds of several thousand RPM's. However, larger locomotive engines typically operate at between 0-1050 RPM. The rate at which the pistons move generally impacts the air intake speed and/or swirl. Lower RPM typically translates into slower air intake. With smaller volume cylinders, sufficient chamber air movement to allow proper atomization of the fuel to air mixture typically occurs during the down stroke. However, larger cylinders typically have much less cylinder air movement which results in a more stagnant trapped air volume. This generally requires a greater fuel injection pressure to be applied to overcome the in-cylinder compression and penetrate the trapped air volume in a sufficiently atomized state, such that entrainment will result in a homogenous and stoichiometric burn of the air/fuel mixture.
Consequently there exists a need for a variable timing fuel injection system for larger compression ignition engines to reduce exhaust emissions, improve fuel consumption and eliminate unnecessary mechanical components and linkages. Such a system should provide high fuel flow rates and high fuel injection pressure to the injectors to allow the engine to operate efficiently.